1. Field of the Invention
This invention generally relates to a battery-charging circuit for a quick charge of a secondary battery.
2. Description of the Relevant Art
Various kinds of secondary batteries are used as power sources for industrial and/or household electrical machines and appliances. The range of applications for these kinds of secondary batteries has now expanded to cover a wider area of use, wherein a single small-sized secondary battery consisting of only one cell (such as, a Ni-Cd battery) or an assembled battery having two or more such batteries connected in series to one another, is used as the power source for a cordless telephone, a video recorder or other household electrical appliances. In such applications, the secondary battery has to be repeatedly charged whenever necessary during use. In general, the most widely used type of this kind of battery charge is a quick or rapid charge carried out with a charging current of 0.5 C or at a higher rate. In this case, the quick charge is controlled so as to avoid possible overcharge of the secondary battery and the charge control system (such as, a -.DELTA.V control system, a temperature control system or the like) is selected for purposes of charge control. After completion of the quick charge, it is a general practice to switch the charge to a trickle charge.
The secondary battery have drawbacks when subjected to a repeated charge and discharge operation for a number of times during its use. The drawbacks include the electrolyte impregnated in a predetermined amount in the battery becoming so greatly decreased so as to cause lack thereof or insufficiency of electrolyte, and the electrodes becoming inactive by oxidation. These conditions bring about an excessive increase in the electric resistance, as well as an excessive decrease in the discharge capacity; thus, making the battery unsuitable for reuse. Even when the battery has become abnormal, as discussed above, it is not possible to determine this abnormal condition from external appearances. In the conventional practice, therefore, even a user does not notice the occurrence of such an abnormality of the battery, and the quick charge is continuously provided thereto under the -.DELTA.V control system and/or the temperature control system or the like for further use as if the battery were normal.
In addition, danger exists in such an abnormal battery in that it could give rise to an unusual and problematic heat generation during the quick charge. Furthermore, when a plurality of batteries (e.g., an assembled battery or a battery pack including a plurality of the second batteries connected in series to one another and packed in a package) is charged through the quick charge under the temperature control system, and is considered to have been fully charged if the temperature control element (in the battery pack near the assembled battery for detecting the battery temperature) detects that the temperature thereof has reached the present level becomes problematic when the battery is reused when in fact it is not reusable.
Therefore, an object of the present invention is to provide a battery charging circuit which is capable of promptly detecting the above-described abnormality of the secondary battery while the quick charge is ongoing; thereby, avoiding the above-described disadvantage of the conventional practice.